Synthesis of well-defined mixed poly(methyl methacrylate)/polystyrene brushes from an asymmetric difunctional initiator-terminated self-assembled monolayer

Article abstract of DOI:10.1021/ma035285p

Well-defined mixed PMMA/PS brushes were successfully synthesized from an asymmetric difunctional initiator-terminated SAM by combining ATRP and NMRP via a two-step process. In situ dehalogenation was carried out by adding n-Bu₃SnH into the reac

Full text of DOI:10.1021/ma035285p

Macromolecules 2003, 36, 8599-8602
8599
mixed initiator-terminated self-assembled monolayers
(SAMs) using a two-step process. Mixed SAMs were
prepared by coadsorption from toluene solutions con-
taining both ATRP-initiator- and NMRP-initiator-
terminated organotrichlorosilanes with predetermined
molar ratios. A similar strategy was also reported by
Ejaz et al. to synthesize binary brushes from mixed
SAMs of two triethoxysilanes.20 Although the mixed
monolayers have been extensively studied for many
years and no macroscopic phase separation has been
reported,21-27 it remains unclear how the two different
species are distributed in the mixed SAMs and to what
extent they are mixed. Preferential adsorption of one
species has been reported (e.g., from the solution con-
taining fluorinated and alkyl trichlorosilanes), but spec-
troscopic data collected by Lagutchev et al. using sum
frequency generation spectroscopy indicated that close
adsorption occurred and well-mixed SAMs were ob-
tained.27 To avoid possible preferential adsorption of one
species from the solutions of two trichlorosilanes and
possible phase separation (in whatever length scale), we
designed and synthesized an asymmetric difunctional
initiator-terminated SAM (Y-SAM) as illustrated in
Scheme 1. The corresponding trichlorosilane is 2-(4-(2′-
oxa-13′-trichlorosilyltridecyl)phenyl)-2-(2′′,2′′,6′′,6′′-tet-
ramethyl-1-piperidinyloxy)ethyl 2-bromo-2-methylpro-
pionate (Y-silane). Y-SAM ensures the mixing of the
ATRP and NMRP initiators at the molecular level and
thus the well-mixing of the two different polymers that
are subsequently grown from them. Y-silane was pre-
pared by a four-step procedure as outlined in Scheme
2. 4-Vinylbenzyl chloride reacted with free radicals
generated from benzoyl peroxide at 80 °C in the pres-
ence of 2,2,6,6-tetramethylpiperidinoxy produced 1 in
a yield of 10%. The reaction between undecylenyl alcohol
and 1 catalyzed by NaH afforded 2, which was then
reacted with 2-bromo-2-methylpropionyl bromide to
incorporate the moiety of an ATRP initiator. Y-silane
was then obtained by hydrosilylation of the precursor
3. Detailed synthetic procedures and characterization
data can be found in the Supporting Information.
Syn th esis of Well-Defin ed Mixed P oly(m eth yl
m eth a cr yla te)/P olystyr en e Br u sh es fr om a n
Asym m etr ic Difu n ction a l
In itia tor -Ter m in a ted Self-Assem bled
Mon ola yer
Bin Zh a o* a n d Ta o He
Department of Chemistry, University of Tennessee at
Knoxville, Knoxville, Tennessee 37996
Received August 31, 2003
Revised Manuscript Received September 28, 2003
The intriguing structures and properties of mixed
homopolymer brushes that consist of two or more
distinct homopolymer chains randomly or alternatively
immobilized on a surface with high grafting densities
have attracted considerable interests in both funda-
mental studies and technology developments over the
past decade.1-19 Theoretical studies have discussed
whether symmetric binary mixed homopolymer brushes
on a planar substrate phase separate laterally, resulting
in a “ripple” state, or vertically, forming a “layered” state
under equilibrium melt conditions, and have predicted
that the “ripple” state should be the one to appear.3,4
Phase separation is expected to occur at a molecular
weight 2.27 times that for the same polymers in a simple
blend at its demixing threshold. The spatial period of
the pattern is predicted to be in the order of the polymer
chain root-mean-square end-to-end distance and to be
independent of the interaction strength between the two
polymer chains.3,4,7 The self-assembly behavior of binary
brushes under exposure to solvents is also of great
interest.7-9 Layered structures may appear in asym-
metric binary polymer brushes.8 Moreover, studies by
Mu¨ller10 and Minko et al.11 have revealed two “dimple”
phases in solvents at large incompatibilities or asym-
metric compositions, where different species form clus-
ters which arrange on a quadratic or hexagonal lattice.
By tuning parameters such as tethering densities, chain
lengths, types of polymer chains, solvents, temperature,
etc., various surface structures and properties could be
achieved by mixed homopolymer brushes. Experimental
studies began very recently, and both “grafting to” and
“grafting from” approaches have been successfully used
to fabricate various binary mixed homopolymer brushes
on flat substrates.11-19 The behavior of the resultant
binary mixed brushes in response to different solvent
treatments has been reported, and the responsive
nature of these surface materials has been demon-
strated. However, the phase behavior of binary mixed
homopolymer brushes under equilibrium melt condi-
tions has not been experimentally investigated. We have
initiated an effort to prepare well-defined binary mixed
homopolymer brushes with controlled molecular weights,
narrow molecular weight distributions, and high, tun-
able grafting densities and to investigate their self-
assembly behavior under various conditions.
Y-SAM was prepared by immersing “Piranha”-treated
silicon wafers in 1.0 mM toluene solutions of Y-silane
at 45 °C typically for 15 h. (It has been reported that
the self-assembly of octadecyltrichlorosilane monolayer
on the oxidized Si (100) surface is initiated by a
mechanism of homogeneous growth at temperatures g
40 °C.28) The advancing and receding contact angles of
water on Y-SAM after cleaning were 84° and 72°,
respectively. Ellipsometry study using a refractive index
of 1.45 for Y-SAM showed that the thickness of Y-SAM
was 17.6 Å, suggesting a monolayer structure. The
samples were then used for the fabrication of mixed
PMMA/PS brushes by sequential ATRP of MMA and
NMRP of styrene (Scheme 1). We chose ATRP to grow
the first polymer and NMRP for the second polymer
because the activation of ATRP initiators is a bimolecu-
lar process, while the free radicals in NMRP are
generated by thermal decomposition which is a unimo-
lecular process.29,30 A unimolecular activation mecha-
nism is preferred for the synthesis of the second type of
polymer chains from the surface because of the steric
hindrance presented by the existing polymer chains.
Surface-initiated ATRP of MMA was carried out at 75
In a previous publication,18 we reported the synthesis
of mixed poly(methyl methacrylate) (PMMA)/poly-
(styrene) (PS) brushes on silicon wafers by combining
atom transfer radical polymerization (ATRP) and ni-
troxide-mediated radical polymerization (NMRP) from
* To whom the correspondence should be addressed. E-mail:
zhao@novell.chem.utk.edu.
10.1021/ma035285p CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/23/2003

8600 Communications to the Editor
Macromolecules, Vol. 36, No. 23, 2003
Sch em e 1. Syn th esis of Mixed P MMA/P S Br u sh es fr om Y-SAM
Sch em e 2. Syn th esis of Y-Sila n e
F igu r e 1. Relationship between the brush thickness and the
molecular weight of “free” PMMA formed in the solutions.
°C using CuBr and N,N,N′,N′,N′′-pentamethyldieth-
ylenetriamine (PMDETA) as catalytic system in the
presence of a “free” initiator, ethyl 2-bromoisobutyrate,
for various lengths of time. It has been reported that
adding a predetermined amount of a “free” initiator into
the reaction mixture helps achieve controlled surface-
initiated polymerizations.18,31 In the previous work,18 we
showed that the NMRP initiator was not activated at
80 °C. Moreover, no noticeable changes were observed
in the thickness and contact angles of a 17.8 nm PMMA
brush that was synthesized from the SAM of (11′-
trichlorosilylundecyl) 2-bromo-2-methylpropionate (si-
lane-4, an ATRP initiator) after the exposure to the
NMRP conditions. A 33.3 nm thick PS film was obtained
from the SAM of 1-(3′-oxa-2′-phenyl-14′-trichlorosilyltet-
radecyloxy)-2,2,6,6-tetramethylpiperidine (silane-5, a
NMRP initiator) in the same pot under the same NMRP
conditions. However, when exposing a SAM of silane-4
and a mixed SAM that was prepared from a toluene
solution containing silane-4 and -5 with a molar ratio
of 54.4:45.6 to the NMRP conditions, we observed a ∼2.0
nm film thickness increase and a water advancing
contact angle of 91°, a characteristic value for PS film,
on the SAM of silane-4, while a 19.1 nm brush was
found from the mixed SAM. Presumably, the C-Br bond
in the ATRP initiator was weak and acted as a chain
transfer agent in NMRP. To eliminate the possible chain
transfers to bromine-terminated PMMA chain ends and
the unreacted ATRP initiator in Y-SAM in the synthesis
of PS by NMRP, tri-n-butyltin hydride (n-Bu3SnH) was
added into the reaction mixtures to in situ remove
bromine atoms after ATRP.32 Control experiments
demonstrated that exposing a SAM of silane-4 and a
F igu r e 2. Relationship between the brush thickness and the
molecular weight of “free” PS formed in the solutions: (1)
mixed PMMA/PS brushes synthesized from a 10.3 nm thick
PMMA brush; (9) mixed brushes synthesized from a 17.0 nm
thick PMMA brush; (2) mixed PMMA/PS brushes synthesized
from a 28.2 nm thick PMMA brush.
10.2 nm PMMA brush grown from Y-SAM to the typical
dehalogenation conditions for 10 and 30 min, respec-
tively,33 completely removed bromine atoms as subse-
quent attempted ATRPs resulted in no changes in the
film thicknesses and water contact angles. We also
confirmed that the NMRP initiator in Y-SAM was stable
under the typical dehalogenation conditions. After de-
halogenation, the silicon wafers with grafted PMMA
chains were extensively washed with a series of solvents
followed by drying with a stream of air. The polymers
formed in the solutions (“free” polymers) were collected
and analyzed by gel permeation chromatography (GPC)
against PS standards. The brush thicknesses were
determined by ellipsometer using the refractive index
of PMMA, 1.49. It was observed that the brush thick-
ness increased with the molecular weight of “free”
PMMA in a linear fashion (Figure 1), suggesting that

Macromolecules, Vol. 36, No. 23, 2003
Communications to the Editor 8601
Ta ble 1. Con ta ct An gles of Wa ter on a Ser ies of Mixed Br u sh es Con sistin g of P MMA a n d P S of Va r iou s Molecu la r
Weigh tsa
b
b
d
d
no.
thickness (nm)
PS Mn
PS Mw
θa (deg)
θr (deg)
θac (deg)
θrc (deg)
θa (deg)
θr (deg)
θae (deg)
θre (deg)
(a) PMMA: Mn of 13 000 and Mw of 15 100 (Thickness ) 10.3 nm)
1
2
3
4
5
6
7
10.3
13.7
19.7
19.9
23.1
25.0
35.1
0
3 500
9 600
10 100
12 800
15 400
23 900
0
4 900
73.5
73
62
63
66
71
75
82
84
75
75
91
91
91
91
91
64
73
74
80
80
84
90
91
62
61
63
64
71
78
82
75
74.5
90
90
91
91
91
64
64.5
71
75
79
80
82
64
74
74
79.5
78
12 800
13 500
16 100
19 300
29 500
80
82
84.5
91
91
74
(b) PMMA: Mn of 21 200 and Mw of 25 000 (Thickness ) 17.0 nm)
1
2
3
4
5
6
7
8
17.0
25.2
27.8
28.4
29.0
32.7
44.5
52.9
0
10 100
12 300
12 500
12 800
15 400
23 900
33 200
0
13 500
16 000
15 900
16 100
19 300
29 500
41 000
73
73.5
74
73
74
84
91
91
62
62
63
64
63
75
83.5
84
75
64
62
65
71
70
82
81
79
74
73
73
74
74
82
91
90
63
58
61
61
62
72
82
83
75
81
83
83.5
85
90.5
91
91
63
62
63.5
69
74
80
81
83
81
84
84.5
84.5
91
91
91
(c) PMMA: Mn of 36 300 and Mw of 43 400 (Thickness ) 28.2 nm)
1
2
3
4
5
6
28.2
35.2
41.5
56.5
68.2
76.5
0
9 600
15 400
26 200
33 200
40 000
0
12 800
19 300
32 700
41 000
50 600
73
73
73
80
87
91
62
75
64
63
61
81
83
82
74
73
74
80
86
91
62
62
57
69
75
82
75
75
75
91.5
91
63
60
61
81
81
81
60
62
68.5
81
84
74.5
75
91.5
91.5
91
91.5
a
We assume the grafted polymers have the same molecular weights and polydispersities as “free” polymers. b The samples were immersed
in CHCl3 for 5 min and then moved out and dried immediately with a stream of air. Water advancing (θa) and receding (θr) contact angles
were recorded. c The samples were annealed in a vacuum at 120 °C for 16 h and cooled to room temperature followed by the contact angle
d
measurements. The samples were treated for the second time with CHCl3 for 5 min at room temperature and then dried with a stream
of air. e The samples were annealed for the second time in a vacuum at 120 °C for 16 h. The standard deviations of contact angle
measurements were <2°.
the polymerizations were well-controlled and the graft-
ing density was almost constant. From the molecular
weight of the polymer chain (Mn) and the brush thick-
ness (d), the average cross-sectional area per chain, A,
can be determined by A ) Mn/(dFNA), where F is the
density (1.2 for PMMA) and NA is Avogadro’s number.
We assume that the grafted polymer chains are well
correlated with the “free” polymers and have the same
molecular weights and polydispersities.31 Calculation
using the point of (22 700 amu, 18.0 nm) in Figure 1
showed that the grafting density was ∼1.8 nm2 per
PMMA chain, indicating that the polymer chains were
highly stretched away from the surface.
To synthesize binary mixed brushes, the silicon
wafers with grafted PMMA brushes were cut into
smaller pieces and exposed to the NMRP conditions at
115 °C in the presence of a “free” initiator, 1-phenyl-1-
(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)-2-benzoyloxy-
ethane, for various amounts of time. After polymeriza-
tions, the samples were exhaustively washed with
CH2Cl2 and CHCl3 to completely remove untethered
polymer chains. The brush thickness increased with the
molecular weight of “free” PS in a nearly linear fashion
in all three sets of samples shown in Figure 2. Consid-
ering that these samples are composed of PMMA and
PS with various molecular weights, it is difficult to
determine the thicknesses by ellipsometry as it is
unclear at this stage at what molecular weights PMMA
and PS chains in the mixed brushes will phase separate
and what kind of structures will form if phase separa-
tion occurs. To estimate the film thickness, we used a
refractive index of 1.49 for all mixed PMMA/PS brushes,
and this might be the reason that the data points of high
molecular weights deviated to some degree from the
linear increase (the refractive index of PS is 1.59).
Calculations using the data points of (9600 amu, 19.7
nm), (10 100 amu, 25.2 nm), and (9600 amu, 35.2 nm)
in Figure 2 showed that the typical grafting densities
were ∼1.6, ∼1.9, and ∼2.2 nm2/PS chain for mixed
brushes synthesized from 10.3, 17.0, and 28.2 nm thick
PMMA brushes, respectively. Steric hindrance should
be responsible for the lower grafting density of PS
chains synthesized from thicker PMMA brushes. Con-
sidering both PS and PMMA chains, the average graft-
ing density was ∼1.0 nm2/chain.
These mixed PMMA/PS brushes exhibited a transi-
tion in water advancing contact angles with increasing
the molecular weight of PS after treatment with CHCl3,
a good solvent for both PS and PMMA (Table 1). When
the molecular weight of PS was lower than that of
PMMA, the brush exhibited a water advancing contact
angle (θa) of ∼74°, a characteristic value for PMMA.
When the molecular weights of the two polymer chains
were comparable, the value of θa was found to be
between 74° and 91°. When the chain length of PS was
longer than that of PMMA, the film exhibited a θa of
91°, implying that the PS chains occupied the air-brush
interface after treatment with CHCl3.
To study the behavior under equilibrium melt condi-
tions, these mixed PMMA/PS brushes were annealed
in a vacuum at 120 °C, above the Tg’s of both PS and
PMMA for 16 h to achieve equilibrium states. After
cooling to room temperature, water advancing and
receding contact angles were recorded, and the data are
summarized in Table 1. When the PS chains were very
short relative to the PMMA chains (e.g., Mn ) 3500 and
Mn ) 9600 in Table 1, parts a and c, respectively), the
surface exhibited a value of θa of 75°, which was the
same as that on a pure PMMA brush. With the increase
of PS molecular weight, the contact angle underwent a
transition from 75° to 91°. However, the transitions
after thermal annealing occurred at lower molecular
weights than those after treatment with CHCl3. For the
samples synthesized from the 10.3 nm thick PMMA

8602 Communications to the Editor
Macromolecules, Vol. 36, No. 23, 2003
(2) Zhao, B.; Brittain, W. J . Prog. Polym. Sci. 2000, 25, 677-
710.
brush (Table 1a), a value of 91° for θa was first found
at PS Mn of 15 400 after treatment with CHCl3 but 9600
after annealing. For the mixed brushes prepared from
the 17.0 nm thick PMMA brush (Table 1b), a value of
91° for θa was first noted at PS Mn of 23 900 after CHCl3
treatment but 15 400 after thermal annealing. For the
samples synthesized from a PMMA brush with a thick-
ness of 28.2 nm, the value of 91° for θa was first observed
at PS Mn of 40 000 after treatment with CHCl3 but
26 200 after thermal treatment. Similar values of water
θa were obtained after the treatment with CHCl3 and
annealing in a vacuum at 120 °C again. These results
suggest more complex phase behavior than has been
predicted by theoretical studies. PS has a lower surface
free energy than PMMA,34 which might favor the
appearance of PS chains at the air-brush interface.
Further investigation is underway.
In summary, we successfully synthesized well-defined
mixed PMMA/PS brushes from an asymmetric difunc-
tional initiator-terminated SAM by combining ATRP
and NMRP via a two-step process. To eliminate the
possibilities of chain transfers to the bromine-termi-
nated PMMA chain ends and unreacted ATRP initiator
in Y-SAM in the second polymerization, in situ deha-
logenation was carried out by adding n-Bu3SnH into the
reaction mixtures. It was observed that the brush
thickness increased with the molecular weight of “free”
polymers collected from the solutions in nearly linear
fashions. We found that the transitions of water θa on
three sets of mixed PMMA/PS brushes with increasing
the molecular weight of PS occurred at lower molecular
weights after thermal annealing in a vacuum at 120 °C
for 16 h than those after treatments with CHCl3.
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Su p p or tin g In for m a tion Ava ila ble: Synthesis of Y-
silane, formation of Y-SAM on silicon wafers, dehalogenation
of an ATRP-initiator-terminated SAM, synthesis of PMMA
brushes from Y-SAM and in situ dehalogenation, attempted
ATRP from the dehalogenated ATRP-initiator-terminated
SAM and PMMA brush, synthesis of polystyrene from the
surface by NMRP, and exposure of 1 to the typical dehaloge-
nation conditions. This material is available free of charge via
the Internet at http://pubs.acs.org.
Refer en ces a n d Notes
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MA035285P